1. Field of the Invention
The present invention relates to gyroscopes, and in particular to planar resonator gyroscopes or inertial sensors and their manufacturing. More particularly, this invention relates to the packaging of isolated planar resonator inertial sensors and gyroscopes.
2. Description of the Related Art
Mechanical gyroscopes are used to determine direction of a moving platform based upon the sensed inertial reaction of an internally moving proof mass. A typical electromechanical gyroscope comprises a suspended proof mass, gyroscope case, pickoffs, torquers and readout electronics. The inertial proof mass is internally suspended from the gyroscope case that is rigidly mounted to the platform and communicates the inertial motion of the platform while otherwise isolating the proof mass from external disturbances. The pickoffs to sense the internal motion of the proof mass, the torquers to maintain or adjust this motion and the readout electronics that must be in close proximity to the proof mass are internally mounted to the case which also provides the electrical feedthrough connections to the platform electronics and power supply. The case also provides a standard mechanical interface to attach and align the gyroscope with the vehicle platform. In various forms gyroscopes are often employed as a critical sensor for vehicles such as aircraft and spacecraft. They are generally useful for navigation or whenever it is necessary to autonomously determine the orientation of a free object.
Older conventional mechanical gyroscopes were very heavy mechanisms by current standards, employing relatively large spinning masses. A number of recent technologies have brought new forms of gyroscopes, including optical gyroscopes such as laser gyroscopes and fiberoptic gyroscopes as well as mechanical vibratory gyroscopes.
Spacecraft generally depend on inertial rate sensing equipment to supplement attitude control. Currently this is often performed with expensive conventional spinning mass gyros (e.g., a Kearfott inertial reference unit) or conventionally-machined vibratory gyroscopes (e.g. a Litton hemispherical resonator gyroscope inertial reference unit). However, both of these are very expensive, large and heavy.
Some symmetric vibratory gyroscopes have been produced, however their vibratory momentum is transferred through their cases directly to the vehicle platform. This transfer or coupling admits external disturbances and energy loss indistinguishable from inertial rate input and hence leads to sensing errors and drift. One example of such a vibratory gyroscope may be found in U.S. Pat. No. 5,894,090 to Tang et al. which describes a symmetric cloverleaf vibratory gyroscope design and is hereby incorporated by reference herein. Other planar tuning fork gyroscopes may achieve a degree of isolation of the vibration from the baseplate, however these gyroscopes lack the vibrational symmetry desirable for tuned operation.
In addition, shell mode gyroscopes, such as the hemispherical resonator gyroscope and the vibrating thin ring gyroscope, are known to have some desirable isolation and vibrational symmetry attributes. However, these designs are not suitable for or have significant limitations with thin planar silicon microfabrication. The hemispherical resonator employs the extensive cylindrical sides of the hemisphere for sensitive electrostatic sensors and effective actuators. However its high aspect ratio and three-dimensional curved geometry is unsuitable for inexpensive thin planar silicon microfabrication. The thin ring gyroscope (e.g., U.S. Pat. No. 6,282,958, which is incorporated by reference herein) while suitable for planar silicon microfabrication, lacks electrostatic sensors and actuators that take advantage of the extensive planar area of the device. Moreover, the case for this gyroscope is not of the same material as the resonator proof mass so that the alignment of the pickoffs and torquers relative to the resonator proof mass change with temperature, resulting in gyroscope drift.
Most recently, some planar resonator gyroscopes devices have been developed (such as a disc resonator gyroscope) which operate through the excitation and sensing of in-plane vibrational modes of a substantially solid planar resonator. These planar resonators obtain enhanced properties over designs such as the hemispherical or shell resonators by enabling greater drive and sensing area in a compact package that is more easily manufactured and packaged. For example, see U.S. Pat. No. 6,944,931 by Shcheglov et al., issued Sep. 20, 2005 and entitled “INTEGRAL RESONATOR GYROSCOPE” and U.S. Pat. No. 7,043,163 by Shcheglov et al., issued May 9, 2006 and entitled “ISOLATED PLANAR GYROSCOPE WITH INTERNAL RADIAL SENSING AND ACTUATION.”
However, such planar resonator gyroscopes may benefit from alternate packaging designs. For example, planar resonator gyroscopes employing embedded capacitive electrodes may be sensitive to distortions arising between their supporting baseplate and planar resonator. Any distortions can affect the capacitive gaps and thus render negative consequences to the operation of the gyroscope, such as damping asymmetry and/or rate drift. Thermal gradients between different structural elements of a planar resonator gyroscope can be a primary contributor to capacitive gap nonuniformity. Conventional microelectronics and microelectromechanical systems (MEMS) manufacturing techniques, which are commonly employed in the development of planar resonator gyroscopes, call for applying a bond across the entire MEMS die to the package substrate. This bond or the package is often a dissimilar material to the MEMS die which can lead to differential expansion versus temperature between the MEMS die and package substrate. In turn, this may result in induced mechanical stress, warpage of the die and internal electrode gap nonuniformity which affect the performance of the gyroscope.
In view of the foregoing, there is a need in the art for improved packaging structures and methods for planar resonator gyroscopes, such as with conventional MEMS packaging techniques. Particularly, there is a need for such structures and methods to reduce thermal expansion differentials, mechanical stress, warpage and capacitive gap nonuniformity. However, there is a need for such structures and methods to be compatible with existing manufacturing methods and materials for planar resonator gyroscopes. As detailed below, the present invention satisfies these and other needs.